Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of rotation in certain quantifiable characteristics of a tire. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During rotation of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity parameters, or attributes, are generally categorized as dimensional or geometric variations (radial run out and lateral run out), mass variance (e.g. mass imbalance), and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Tire uniformity measurements, such as static balance measurements, can result from manufacturing effects that have both tire effects and process effects. Examples of tire effects include effects due to tire material components (e.g., the product start point or joint overlap location of one or more of casing textile plies, belt plies, bead rings, inner liner, tread and other rubber layers of the tires), manufacturing techniques (e.g., the relative location in which a green tire is introduced on a building drum, placed into a mold or curing press, and the like), and/or controllable conditions used in the tire construction process (e.g., the temperature and pressure at which green tires are subjected during the curing process or other manufacturing steps.) Examples of process effects may arise from such manufacturing conditions as a roller influence, extruder surge, fluctuation in a process condition (e.g., temperature, pressure, speed, etc.) and others.
The impact of tire effects and process effects within tire uniformity measurements are respectively represented by “tire harmonic” or “process harmonic” components of the composite uniformity measurement. A tire harmonic component has a period that fits an integer number of times within the tire circumference. A process harmonic component has a period that does not fit an integer number of times within the tire circumference.
One uniformity parameter directed to mass variance is static balance. A static balance measurement can provide a measure of mass imbalance of a tire. In particular, the static balance measurement can represent the first harmonic of the mass imbalance about the tire. Static balance measurements can be acquired by a static balance machine where a tire can be placed on its vertical axis. Gravity causes the portion of the tire with the greatest mass to deflect downward. The magnitude and azimuthal location of the deflection can provide a measurement of the static balance of the tire. The static balance can be represented as a vector with the magnitude determined based at least in part on the amount of deflection and the phase angle determined from the azimuthal location of the deflection.
In many practical cases, only the magnitude of the static balance measurement is acquired and/or stored in a memory for future analysis. For instance, determination of the azimuthal location may require that a barcode or other indicator is attached to the tire during its manufacture to act as a reference point for measurement of the uniformity parameter. If this capability is absent, then the azimuthal location of the deflection can be difficult to determine.
Process harmonics can cause static balance measurements to vary from tire to tire depending on the particular pattern and rate of introduction of the process harmonic. For instance, a static balance measurement for a first tire can have a different magnitude and azimuthal location than a static balance measurement in a second tire manufactured according to the same manufacturing process. It can be desirable to derive process harmonic information from a sequence of static balance measurements for a set of tires to drive process improvement and correction efforts.
Thus, a need exists for a system and method for estimating magnitudes of process harmonics from a sequence of the static balance measurements for a set of tires. A system and method that can estimate magnitudes of process harmonics without requiring azimuthal or phase angle information for the static balance measurements would be particularly useful.